The NTSB has released a preliminary report on the Tesla Model S crash in Mountain View, California on March 23 that killed one person and led to speculation about whether Tesla's Autopilot system may have been at fault.
We discussed this previously on Cake in Tesla Autopilot Was Engaged in Fatal Crash.
I found these details from the report particularly interesting:
• At 8 seconds prior to the crash, the Tesla was following a lead vehicle and was traveling about 65 mph.
• At 7 seconds prior to the crash, the Tesla began a left steering movement while following a lead vehicle.
• At 4 seconds prior to the crash, the Tesla was no longer following a lead vehicle.
• At 3 seconds prior to the crash and up to the time of impact with the crash attenuator, the Tesla’s speed increased from 62 to 70.8 mph, with no precrash braking or evasive steering movement detected.
Knowing what we know about the site of the accident (see the image below), this paints an interesting picture of what may have been the cause.
This is only my own theory, but here's what I think may have happened:
The Tesla was driving south in the left lane of US-101 with Autopilot engaged. Since there was another car in front of it, Autopilot was matching that car's speed and using its position as a high probability indicator of where the lane was going (it would have used lane markings as a slightly lower probability indicator).
While approaching the US-101/SH-85 interchange, the lead vehicle moved into the left-side exit lane to merge onto SH-85. Autopilot briefly steered left to follow the lead vehicle before realizing that it was moving into a new lane, at which point it stopped following the lead vehicle and went back into lane-keeping mode using the lane lines on the road as its primary indicators.
My theory is that at this point, the Tesla had actually followed the lead vehicle over the faded left lane line of US-101's left lane and into the gore lane, then saw the much clearer line between the gore lane and the SH-85 ramp. It was at this point that Autopilot stopped following the lead vehicle and chose to remain in its lane, but since the gore lane had no cross-hatch markings or any other clear indications that it wasn't a normal traffic lane, Autopilot didn't realize it was no longer in a traffic lane.
Since it was set to maintain a speed of 75 mph and there was no longer a car ahead, Autopilot sped up. It didn't detect the crash attenuator because it was a small, narrow, fixed object — something that's notoriously difficult for a self-driving system to reliably detect in the distance. The driver also apparently failed to see the crash attenuator and didn't take evasive action, and the car hit the barrier. Normally the crash attenuator would have absorbed a significant amount of the impact force, possibly saving the driver's life, but it had already been damaged in a previous accident and Caltrans hadn't repaired it.
This strikes me as a classic "perfect storm" event: a sequence of cascading failures culminating in a crash, just like we tend to see in airline accidents.
If Autopilot hadn't been following a lead vehicle, or if the lead vehicle hadn't exited, or if the lane markings hadn't been faded, or if Caltrans had painted cross hatch markings in the gore lane, or if Caltrans hadn't failed to repair the crash attenuator, or if Autopilot had been capable of detecting the barrier, or if the driver had been paying more attention, then the accident might not have happened (or at least the driver might have survived).
The good news is there's a lot we can learn from this. Tesla should improve Autopilot's lane detection algorithms. Caltrans should improve their maintenance practices and paint more visible markings at gore points. And above all, Tesla owners should pay more attention to the road while using Autopilot.